1. Field of the Invention
The present invention relates to a hydraulic brake apparatus for a vehicle, and more particularly to the apparatus having an auxiliary pressure source for generating a power pressure and a valve assembly for regulating the power pressure into a regulated pressure.
2. Description of the Related Arts
As for the hydraulic brake apparatus having the auxiliary pressure source and the valve assembly, the U.S. Pat. No. 3,928,970 discloses power-assisted master cylinder assemblies which comprise a piston movable in a cylinder bore, a pressure space ahead of the piston having an outlet for connection to a brake circuit, a power chamber behind the piston having an inlet for connection to a source of fluid under pressure, and a control valve assembly for controlling the supply of fluid from the pressure source to the power chamber. It is an object of the invention disclosed in the above United States Patent to provide a power-assisted master cylinder assembly in which the piston is pedal operated but which does not require a movable inlet or inlet seals which move under pressure. According to the assembly as disclosed above, the power-assisted master cylinder includes a pedal operated first piston for pressurizing the pressure space connected to the brake circuit, and a second piston for operating valve means controlling the fluid assisting the pedal, movement of the first piston being transmitted to the second piston either hydraulically or mechanically. In the assembly, one end of the second piston is exposed to the pressure in the pressure space and the other end is exposed to the pressure of the assisting fluid.
In the UK Patent Application GB2170874 discloses servo-assisted master cylinder assemblies having a master cylinder with primary and secondary pistons, with a servo chamber defined behind the primary piston, and having valve means provided with a spool for controlling pressurization of the servo chamber from a source of pressure fluid.
According to the assemblies as disclosed in the above publications, the control valve (or regulator valve) is controlled by the master cylinder pressure. When the wheel cylinder pressure is increased or decreased in response to depression of the brake pedal, the control valve is actuated to assist the operation of the brake pedal after the wheel cylinder pressure was increased or decreased by the master cylinder. Therefore, even if the brake pedal is released during the braking operation, for example, the pressure in the power chamber can not be decreased immediately after the brake pedal was released, because the frictional force of sealing members mounted on the master piston is large. Only after the depressing force to the brake pedal was reduced to overcome the frictional force of sealing members, the pressure comes to be reduced. As far as conventional resilient sealing members are used, the frictional force is likely to exceed a limit to cause a delay in pressure decreasing operation.